In the fields of optical communications or optical measurements, optical modulators have been widely used. Particularly, with the development of multimedia, the amount of information being transmitted tends to increase and there is a demand for enlargement in the modulation frequency bandwidth of optical modulators. As one means for fulfilling the demand, an external modulation system as a waveguide type optical modulator in which an optical waveguide or a modulation electrode is formed on a substrate formed of lithium niobate (LN) or the like having an electro-optical effect has been employed and the diversification thereof has proceeded. For the purpose of the enlargement in frequency bandwidth in the external modulation system, it is necessary to accomplish the rate matching of a microwave as a modulation signal with an optical wave and a decrease in driving voltage.
Accordingly, by reducing the thickness of the substrate having an electro-optical effect, attempts have been made to satisfy the rate matching condition of a micro wave with an optical wave and to simultaneously reduce the driving voltage.
In Patent Citations 1 and 2, an optical waveguide and a modulation electrode are formed in a thin substrate with a thickness of 30 μm or less and a reinforcement plate with a lower dielectric constant than that of the substrate is bonded to the substrate to lower the effective refractive index with respect to microwaves, thereby accomplishing the rate matching of the microwaves with optical waves and enhancing the mechanical strength of the substrate.    Patent Citation 1: Japanese Patent Application Laid-Open No. 64-18121    Patent Citation 2: Japanese Patent Application Laid-Open No. 2003-215519
However, in a wide-bandwidth optical modulator, for example, an optical modulator corresponding to 40 GHz, when the optical input power input to the optical modulator is enhanced for the purpose of transmission over a long distance, problems occur such as a decrease in extinction ratio, an increase in optical loss, and a variation in bias point. Particularly, when the optical input power is equal to or greater than 10 mW, these problems become marked. As a result of extensive research, the inventors found that the problems are caused by signal light propagating in the optical waveguide interfering with stray light generated in an input portion for inputting a laser beam to the optical modulator, the optical waveguide in the optical modulator causing a photorefractive phenomenon, and a grating being formed in the optical waveguide portion. The grating formed in the optical waveguide portion returns the signal light traveling in the optical waveguide in the opposite direction of the traveling direction or reflects the signal light to the outside of the optical waveguide, thereby causing a decrease in extinction ratio of the signal light. The decrease in extinction ratio is also caused because the stray light is coupled to the signal light propagated in the optical waveguide.
The photorefractive phenomenon is a phenomenon in which the refractive index of a material varies due to contact with light. Specifically, when a spatial light intensity distribution is caused by optical interference on the basis of the characteristic that charges in the material migrate due to the light, the charges are re-distributed due to the light intensity distribution and the internal electric field varies locally due to the eccentric distribution of charges. Since the internal electric field changes the refractive index of a material, the refractive index distribution of the material corresponding to the light intensity distribution is formed as a result.
Since the photorefractive phenomenon has a characteristic that the refractive index gradually varies and the scattering gains strength with the passing of time when the material is in continuous contact with light, the deterioration of the characteristics of the optical modulator such as the decrease in extinction ratio and the increase in optical loss is marked after driving the optical modulator for a long time.
As means for solving such problems, Patent Citation 3 discloses that stray light removing means is disposed on the surface of a substrate to suppress the decrease in extinction ratio and the increase in optical loss due to the photorefractive phenomenon in the optical waveguide.    Patent Citation 3: Japanese Patent Application Laid-Open No. 2004-93905
On the other hand, when the thickness of the substrate is set to 30 μm or less or 20 μm or less as in Patent Citations 1 and 2, the density of the stray light confined in the substrate increases and it is thus not possible to satisfactorily suppress the deterioration in characteristics of the optical modulator such as the decrease in extinction ratio and the increase in optical loss only by the use of the stray light removing means described in Patent Citation 3, compared with the optical waveguide type modulator with a thickness of a substrate of about 1 to 0.5 mm. In a Mach-Zehnder waveguide, a section (input branching portion) until branched waveguides obtained by branching an optical waveguide are guided to a parallel orientation or a section (output merging portion) until the parallel branched waveguides are merged is gradually curved (with the minimum radius of curvature of about 150 mm) so as to cause optical waves not to leak from the optical waveguide. Accordingly, such a portion often comes in contact with stray light and thus is much influenced by the photorefractive phenomenon. When the length of the portion (operating portion) of the optical waveguide on which the electric field formed by a modulation electrode acts increases, the photorefractive phenomenon has a great influence on the optical waveguide. Accordingly, reduction of the influence of the photorefractive phenomenon on the Mach-Zehnder waveguide type optical modulator is an important issue.
When the Mach-Zehnder waveguide is formed in a thin plate, the optical waves propagated in the branched waveguides are flattened by the light confinement action of the thin plate. Accordingly, to avoid crosstalk between the branched waveguides, it is necessary to increase the distance between the branched waveguides, whereby the length of the input branching portion or the output merging portion further increases. This means that the influence of the photorefractive phenomenon is greater.